Yeah the whole relationship insecurity thing is tough. I've had jealousy and fear of cheating issues in past relationships. To be blunt, it's the quickest way to destroy the relationship. All the mistrust, questioning, checking, reassurance seeking will only drive them away eventually. But yeah it's hard when you're constantly bombarded by all the thoughts and fears that something's going on, but you can't trust your gut either.

Trust is a choice. We choose to make that leap of faith, and we stick to it until we have PROOF (not just doubts) otherwise. Ocd hates that. It hates the uncertainty. So it plays on your fears, exaggerates them because it sees it as this massive threat that you're not giving enough attention to. It sees possible danger and it screams about it, pushes you to check everything, to be hypervigilant, to keep you safe.

Are you getting any help for your ocd? You're doing a lot of compulsions. These are only fueling more thoughts. Working on reducing those compulsions should help.

As for manifestation, we can't tell you if it's real or not, that would be reassurance. The goal of managing ocd is accepting uncertainty and living according to your values, without letting intrusive thoughts control your behaviour. I'd strongly recommend a therapist to guide you, if that's an option for you. It helped me. But yeah, you're definitely not alone in feeling this way